In automobile exhaust systems, it is advantageous to introduce air directly into the exhaust manifold or catalytic converter to assist in oxidizing the unburnt gaseous components contained in the exhaust gas. The air is supplied by an air pump, which is driven by the automobile engine and is therefore operable during the operation of the engine. A check valve is usually provided in the air supply conduit leading from the air pump to the catalytic converter or exhaust manifold to protect the air pump from hot, corrosive exhaust gas which may backflow into the air supply conduit.
The presence of air in the air supply conduit downstream of the check valve and upstream of the exhaust manifold or catalytic converter generally provides an air cushion which prevents the high-temperature exhaust gas from coming into contact with the check valve. Nevertheless, when the air pump is not operating, the air cushion dissipates and exhaust gas may seep into the air supply conduit and come into contact with the check valve. The exhaust gas is of relatively high temperature Therefore, check valves are typically made of heat resistant metals. However, the metal to metal contacts between the parts of these check valves are insufficient to adequately seal the check valve and prevent leakage of air.
Furthermore, as the air cushion between the check valve and the source of exhaust gas diminishes, due to leakage in the check valve, exhaust gas is allowed to come into contact with the metal parts of the check valve. As a result, vaporized water and harmful gaseous components contained in the exhaust gas, such as nitrogen oxide and sulfur, condense on the metal parts, forming such corrosive compounds as sulfuric acid. The effect of the corrosive compounds on the metal check valves greatly reduce their ability to prevent a backflow of exhaust gas from reaching the air pump.
One prior device, described in U.S. Pat. No. 3,871,175, attempts to solve this problem by providing two check valves in the air supply conduit: a metal check valve adjacent the source of high-temperature exhaust gas, and a plastic check valve adjacent the air pump. Such a system is obviously more costly than a single check valve system, and fails to solve the problem of providing a single check valve which is capable of effectively protecting the air pump from a backflow of exhaust gas.
To overcome the problems caused by corrosion in metal check valves, a plastic check valve is sometimes used. Typical plastic valve housings are comparatively weaker than metal housings, though, and tend to fracture along the seams joining the several sections. In addition, although constructed of high temperature plastic, the internal parts of these check valves may still be affected by the heat of the exhaust gas. For example, a valve seat subjected to high temperature may deform, resulting in an incomplete seal between the valve closure element and the valve seat. In addition, the internal parts of existing valves do not sealingly engage to provide a sufficient seal against the backflow of exhaust gases. Furthermore, the orifices of existing valve seats are so large that the elastomeric sealing member often bends into the orifices when exposed to back pressure, thereby resulting in an inadequate seal. As a result, the check valve is unable to prevent a backflow of exhaust gas from reaching the air pump.